Hydraulic system including flow stabilization means

ABSTRACT

A HYDRAULIC SYSTEM INCLUDING A HYDRAULIC MOTOR WITH MEANS FOR CONTROLLING THE PRESSURE IN THE INTAKE LINE OF THE MOTOR TO PREVENT CAVITATION AND ELASTIC COMPRESSION OF THE HYDRAULIC OIL WHEN THE LOAD ON THE MOTOR INCREASES.

Dec. 7, 1971 HERNDQN, JR 3,625,007

HYDRAULIC SYSTEM INCLUDING FLOW STABILIZATION MEANS Filed Feb. 7. 1969 2 Sheets-Sheet 1 fiee 39y Herndon BY ATTORNEY 1971 L. R. HERNDON, JR 3,

HYDRAULIC SYSTEM INCLUDING FLOW STABILIZATION MEANS Filed Feb. 7, 1969 2 Sheets-Sheet 2 INVI'IN'IHR 3,625,007 HYDRAULIC SYSTEM INCLUDING FLOW STABILIZATION MEANS Lee R. Herndon, Jr., 5640 Woodward Ave., Birmingham, Mich. 48011 Filed Feb. 7, 1969, Ser. No. 797,569 Int. Cl. FlSb 13/042; Folk 13/02 U.S. Cl. 60105 6 Claims ABSTRACT OF THE DISCLOSURE A hydraulic system including a hydraulic motor with means for controlling the pressure in the intake line of the motor to prevent cavitation and elastic compression of the hydraulic oil when the load on the motor increases.

This invention relates to hydraulic systems, and is particularly concerned with hydraulic systems including a hydraulic motor driven by flow of hydraulic fluid from a source such as a variable displacement pump, and having means for stabilizing the flow through the motor such that both cavitation and the effects of elastic compression of the hydraulic fluid are avoided,

In such hydraulic systems it is necessary to maintain a minimum pressure on the intake side of the hydraulic motor for preventing cavitation which can occur due to the tendency of the inertial forces on the motor to cause the motor to run away in the event that there is a reduction in the amount of hydraulic oil available to the motor. When cavitation occurs, the motor runs faster than called for by the oil supply due to inertia and attempts to pull oil through the motor faster than it is being supplied to the motor. In order to prevent cavitation, it is necessary to maintain a minimum pressure on the intake side of the motor so that sufficient hydraulic fluid is available to the motor corresponding to the actual speed of the motor.

Another frequently recurring problem in such hydraulic systems is that of power loss due to compression of the hydraulic fluid on the intake side of the hydraulic motor when there is a load increase on the motor. The load increase may momentarily slow the motor which tends to cause a pressure increase on the intake side of the motor. In hydrostatic transmissions of the type including a variable displacement pump for driving a constant displacement motor, it is desirable to maintain the speed of the output shaft of the motor in accordance with the displacement of the pump. When the pump displacement increases, there should be a corresponding increase in the speed of the motor shaft. However, when the load on the motor increases, a back pressure occurs between the intake side of the motor and the output side of the pump which can, due to the length of the passages involved, cause a substantial elastic compression of the hydraulic oil with the result that there is a lag in the speed of the motor shaft, and the motor speed no longer corresponds to the displacement of the pump. The amount of lag, or loss in r.p.m., increases with the length of the passageways between the pump and the motor due to the greater amount of compression that is possible.

One of the objects of this invention is to reduce the likelihood of the occurrence of cavitation in a hydraulic system having a hydraulic motor driven by the flow of hydraulic fluid through the motor.

Another object of this invention is to prevent compression of hydraulic oil on the intake side of a hydraulic motor caused by increases in the load on the motor.

A more specific object of this invention is to provide a hydraulic system having a source of hydraulic fluid United States Patent for supplying hydraulic fluid to a hydraulic motor with means defining a path of flow from the source to the motor including an intake line for the motor and means defining a path of flow from the motor including an output line for the motor with means for maintaining the pressure in the intake line at a normal value including flow control means controlling one of the paths operable in response to pressure changes in the intake line to change the pressure in said one path until the intake pressure returns to normal.

Another object of this invention is to prevent cavitation in a hydraulic system including a hydraulic motor by connecting the intake line of the hydraulic motor with an auxiliary path of flow from a source when the pressure in the intake line decreases below a minimum.

Still another object is to prevent elastic compression of the hydraulic oil on the intake side of a hydraulic motor by decreasing the pressure in the output line from the motor when the pressure on the intake side of the motor increases.

Yet another object is to prevent cavitation and elastic compression in the intake line of a hydraulic motor by maintaining a constant pressure in the intake line.

The foregoing, and other objects, are achieved by the provision of a hydraulic system including a hydraulic motor with means defining a path of flow from a source of hydraulic fluid to the motor including an intake line for the motor and means defining a path of flow from the motor including an output line for the motor. Flow control means is operable in response to pressure changes in the intake line from a normal value to vary the pressure in one of the paths until the intake pressure returns to the normal value. A port in one of the paths is controlled by flow control means in such a manner that the port is open and closed in accordance with the position of the flow control means relative to the port, and the flow control means is movable in response to pressure changes in the intake line to open and close the port.

In one embodiment of the invention a spool valve increases and decreases the size of a discharge port for the output line of the hydraulic motor in accordance with changes in the pressure in the intake line. The spool valve is urged in a direction to increase the size of the discharge port by the intake pressure and is urged in a direction to decrease the size of the discharge port by a pilot pressure. When the load on the motor increases, the resulting increase in the pressure in the intake line causes the spool valve to increase the size of the discharge opening to correspondingly reduce the pressure in the output line of the motor until the pressure in the intake line returns to normal,

-In another embodiment of the invention, a hydraulic motor has its intake line connected with a source of hydraulic fluid through a primary path of flow. The intake line of the motor is also connected in an auxiliary path of flow which is controlled by a modulating spool valve which operates to connect the intake line with the auxiliary path of flow from the source when the pressure on the intake side of the motor decreases below normal, or the minimum pressure required to prevent cavitation. Thus, when the pressure on the intake line to the motor decreases to a point where cavitation might occur, the spool valve connects the intake line with the auxiliary path of flow from the source to supply additional oil to the intake line and restore the intake pressure to normal.

Other objects, advantages and features of the inventian will become apparent from the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a first embodiment of the invention; and

FIG. 2 is a schematic diagram of a second embodiment of the invention.

In FIG. 1, reference numeral 2 designates a variable capacity pump for providing a source of hydraulic fluid. Hydraulic lines 4 and 6 extend from opposite sides of pump 2. Pump 2 may be reversible, and when the pump is driven to deliver through hydraulic line 4, line 6 serves as a return line to the pump, and vice versa. Hydraulic lines 8 and 10 each have one of their ends respectively connected with lines 4 and 6 and their other ends respectively connected with lines 16 and 18. Check valves 12 and 14 prevent flow of hydraulic fluid from lines 16 and 18, respectively, to lines 8 and 10. Lines 16 and 18 are connected with opposite sides of a hydraulic motor 20 of conventional construction to be driven by fluid delivered from pump 2. Hydraulic motor 20 is reversible so that when the pump delivers in one direction, line 16 serves as an intake line for the motor and line 18 serves as an output line for the motor, and vice versa. Reference numerals 22 and 24 indicate high pressure relief valves for relieving excessive pressure in lines 16 and 18, respectively.

Lines 16 and 18 communicate with discharge ports 26 and 28, respectively, in a cavity 30 defined in a valve casing 32. Lines 4 and 6 communicate with return ports 34 and 36, respectively, in the cavity 30. Slidably received in the cavity 30 is flow stabilization or control means in the form of a spool valve designated generally by reference numeral 38 and formed with lands or pistons 40, 42 and 44 projecting from a stem 46.

The hydraulic line 6 is connected through a line 48 with an inlet port 52 for an inlet chamber 54 defined by land and the end wall 56 of cavity 30. A check valvein line 48 prevents flow from line 48 to line 6 but permits flow in the opposite direction. Line 4 is connected with line 48 through a line 58 controlled by a check valve 60 which permits flow only from line 4 into line 48. It is apparent that the inlet chamber 54 is subjected to the higher of the pressures from lines 4 and 6.

A central passage 62 extends through the stem 46 of spool valve 38 and communicates through a restricted port or orifice 64 with a pilot chamber 66 defined by land 44 and end wall 68 of cavity 30. Pilot chamber 66 has an exhaust port which communicates through a line 70 with a port 72 in a pilot valve casing, and port 72 in turn is con trolled by a pilot valve 74. When the pilot valve 74 is opened to permit flow from line 70 through port 72, the fluid flows around valve 74 through an exhaust line 76 to sump. The closing force on pilot valve 74, and thus the pressure in pilot chamber 66, is determined by the pressure in a pilot valve chamber 78. Pilot valve chamber 78' has a pressure responsive movable wall in the form of a piston member 80 which slidably supports the pilot valve 74 and is engageable with a stop member 82. Pilot valve 74 has a stem slidably mounted in a cylindrical bore formed in piston 80 and is biased into engagement with port 72 by a spring 84. When piston 80 engages stop '82, spring 84 applies a maximum closing force on the pilot valve 74, which in turn determines the maximum pilot pressure. Increases in pressure in pilot valve chamber 78 thus increase the closing force on valve 74 and correspondingly cause an increase in the pressure in pilot chamber 66. The pilot valve chamber 78 receives fluid from a line 86 connected with a port 87 in cavity 30. Port 87 registers with a peripheral groove 88 in land 44 which in turn is in communication with passage 62. Thus, fluid flows from the inlet chamber 54 through passage 62 in the groove '88, and from groove 88 through port 87 and line 86 into the pilot valve chamber 78. Pilot valve chamber 78 is connected with exhaust through a restricted port 90. A spring 92 urges the spool valve 38 toward the right in FIG. 1.

Assuming that pump 2 is driven to deliver hydraulic fluid through line 4, the pump constitutes a source of hydraulic fluid and lines 4 and 8 as well as the intake line 16 defines a path of flow from the pump to motor 20, and a path of flow from motor 20 including output line 18 is defined by the space between lands 42 and 44 and line 6. The flow control means or spool valve 38 is operable in response to pressure changes in the intake line to change the pressure in the path of flow from the motor 20 by increasing and decreasing the size of the discharge port 28 through which the fluid flows along the path of flow from the motor until the intake pressure is returned to normal. The discharge port 28 is controlled by the flow control means 38 in such a manner that port 28 is opened and closed in accordance with the position of land 44 of the flow control means relative to the port 28. The flow control means, or spool valve 38 is movable in response to pressure changes in the intake line to open and close port 28. The flow control means is urged toward the left by the pressure in the intake line, which is the same as the pressure in the inlet chamber 54, and is urged in a direction to close discharge port 28 by the pilot pressure in pilot chamber 66. The pilot pressure is determined in accordance with the position of the spool valve 30 is determined by the rate of flow from the pump 2 since the pilot valve chamber 78 has communication with the path of flow from the source through the port 87. The pilot valve 74 controls communication between the pilot chamber and exhaust, and the pilot valve is urged to a closed position against the pilot chamber pressure through line by the pressure in the path of flow from pump 2, which is the same as the pressure in the pilot valve chamber 78.

The flow control means, which comprises the spool valve 38, is received in cavity 30 in such a manner that the first land 40 cooperates with the end wall 56 of cavity 30 to define the inlet chamber 54, and the second land 44 cooperates with the second end wall 68 of cavity 30 to define the pilot chamber 66.

When the pump starts into operation, hydraulic fluid is delivered through line 4 to line 8 and past check valve 12 into the intake line 16 from motor 20. The hydraulic fluid flows through the motor into the output line 18 and from output line 18 through discharge port 28 into the space between lands 42 and 44, and through return port 36 and return line 6 to pump 2. The pressure in the inlet chamber 5,4 is the same as the pressure in line 4 and this pressure urges the spool valve toward the left to uncover the discharge port 28 sufliciently to permit fluid to be discharged from output line 18 into cavity 30 between lands 42 and 44. The fluid from inlet chamber 54 also passes through passage 62 into the pilot valve chamber 78 through groove 88 and line 86, and through the restricted port 64 into the pilot chamber 66. Since the pilot valve chamber 78 has restricted communication only with sump through port 90, the pressure in pilot valve chamber 78 is sub stantially the same as that in the inlet chamber 54 and in the path of flow from the source 2 to the motor 20. An increase in the load on motor 20 will cause a pressure rise in intake line 16 which in turn will cause a corresponding increase in the pressure in line 8 and inlet charm ber 54. The increased pressure in inlet chamber 54 will move the spool valve 38 to the left to increase the size of the opening through discharge port 28 and thus correspondingly reduce the resistance to flow from output line 18 and thus the pressure in line 18. The drop in pressure in output line 18 will thus cause a corresponding reduction in pressure in intake line 16 to thereby maintain a constant pressure in line 4 to prevent the occurrence of elastic compression of the hydraulic oil in the path of flow from pump 2 to motor 20.

When it is desired to run the motor at high speeds, the pressure in the line 4 will tend to rise sufiiciently to cause the pressure in inlet chamber 54 to urge valve 38 toward the left sufficiently to move the groove 88 out of registry with port 87 so that flow from output line 18 will have the minimum amount of resistance. However, for low speeds, the groove 88 will be in communication with port 87 throughout the range of movement of valve 38 caused by variations in the pressure in the intake line. Thus at low speeds, the pilot valve chamber 78 will be in com munication with groove 88 and the pilot valve will be maintained at its high setting due to the engagement of piston 80 with stop 82. At high speeds, groove 88 will move to the left out of communication with port 87 and piston 80 will seat on the lower wall of chamber 78 to reduce the spring force on pilot valve 74 and place the pilot valve 74 at its low pressure setting. The spool valve 38 controls cavity 30 to thus constitute flow stabilization means for preventing elastic compression of hydraulic fluid in the intake line of the hydraulic motor by decreasing the pressure in the output line of the hydraulic motor in response to increases in the pressure in the intake line above a predetermined maximum pressure.

When the pump 2, and hence motor 20 is reversed, line 1 6 will become the delivery line, line 4 will become the return line for the pump, and line 18 will become the,

intake line for motor 20 and line 16 will become the output line for the motor 20. In this case, a discharge port 26 will be under the control of land 42 and will be opened and closed in accordance with changes inthe pressure in inlet chamber 54. The pressure in inlet chamber 54 will be the same as the pressure in the delivery line 6.

With reference to the embodiment of FIG. 2, reference numeral 102 designates a pump having a delivery line 104 which is connected with a servo valve casing 106 having a cavity receiving a servo valve 108. The servo valve 108;- includes a stem 110 and lands 112, 114, 116 and 118. A return line 120 communicates with the servo valve cavity between lands 112 and 114, and a return line 122 communicates with the cavity between lands 116 and 118, and with a sump or reservoir at 24. The return line 120 communicates with the return line 122.

One end of a line 126 is connected with a port 125 controlled by land 116, and the other end of line 126 is connected with a line 128 extending from one side of a hydraulic motor 130. Similarly, one end of a line 132 communicates with a port 131 controlled by land 114 and the other end of line 132 is connected with a line 134 extending from the opposite side of the hydraulic motor 130. A line 136 has one end connected with a port communicating with the space between lands 114 and 116, and the other end of line 136 is connected with a line 138 having one end portion 140 connected with a valve cavity 144 in a valve casing 148, and the other end 142 communicating with a valve cavity 146 defined by a valve casing 150. Slidably received in the valve cavity 144 is a flow control means in the form of a spool valve 152 having lands 154 and 156. Similarly, a spool valve 158 is slidably received in cavity 146 and includes lands 160 and 162.

One end of a line 164 is connected with a port 163 communicating with cavity 144, and the other end of line 164 is connected with line 128. Similarly, one end of a line 166 is connected with a port 165 in cavity 146, and the other end of line 166 is connected with line 134. Line 128 communicates with a port 129 in the end wall 168 of casing 148, and line 134 is connected with a port 133 in the end wall 170 of casing 150. Valve 152 is urged toward the left of FIG. 2 by a spring 172 seated between land 154 and the end wall 173 of casing 148, and a similar spring 174 seated between land 162 and the end wall 175 of casing 150 urges the spool valve 158 toward the right in FIG. 2. The space between land 154 and end wall 173 is connected with an exhaust line 176 to a sump or reservoir at 177. Similarly, an exhaust line 178 connects the space between land 162 and end wall 175 of casing 150 with the exhaust at 179.

Pressure source 102 delivers hydraulic fluid through delivery line 104 into the space between lands 114 and 116. The fluid flows from line 136 to the cavities 144 and 136. When the servo valve 108 is shifted to the right in FIG. 2, the hydraulic fluid flows through port 125 to line 126 and line 128, and the fluid flows from the motor through lines 134 and 132 to port 131 and from port 131 into the space between lands 112 and 114 to return line 120. In

this condition, line 128 is the intake line for motor and line 134 is the output line for motor 130. The pressure in the intake line 128 acts against land 156 of the spool valve 152 to urge the valve toward the right in FIG. 2 and close port 163. The pressure in line 128 tends to close port 163 and spring 172 tends to urge the valve 152 in a direction to open port 163 and connect line 128 with the flow from line 140 through line 164.

In the FIG. 2 embodiment, the path of flow from the source, i.e., pump 102, to motor includes a primary path of flow through line 126 to the intake line 128, and an auxiliary path of flow through lines 136, 138, cavity 144 and line 164 to the intake line 128, with port 163 located in the auxiliary path of flow and being controlled by the flow control means in the form of spool valve 152. Port 163 connects the valve cavity with the intake line through line 164 and constitutes an outlet port for valve cavity 144. The spool valve 152 controls communication between the outlet port 163 and the inlet port 141 communicating with the line 140. Port 129 connected with the end of intake line 128 constitutes a sensing port communicating with the intake line, the land 156 being exposed to the pressure of the sensing port on a side opposite the second land 154. The port connecting line 176 with the cavity 144 constitutes a reference port to provide a fixed reference pressure or force, together with spring 172 acting on land 154 so that the spool valve is movable in the cavity in,response to changes in the pressure in the intake line.

When the system is operating normally, port 163 is closed and the intake line 128 is supplied through the primary path from line 126 past the land 116 of the servo valve 108, which is in a fixed position. If for any reason the pressure in the intake line 128 should fall, the pressure at port 129 will correspondingly decrease and spool valve 152 will shift to the left and open port 163 to change the pressure in the path of flow from the source by increasing the area through which fluid flows from the pump 102 to the motor 130 and supplying additional oil to intake line 128. When port 163 is opened, fluid flows from the auxiliary path into the intake line 128 to increase the pressure in line 128 and prevent cavitation. The system is reversed by shifting the servo valve 108 toward the left so that fluid is supplied to the motor through line 132 from port 13 1 instead of through line 126, and is exhausted or returned to sump 124 through line 122 instead of through line 120.

It is, of course, within the scope of the invention to provide a separate source of hydraulic fluid for the auxiliary path of flow through lines 136, 138, casings 148 or 150 and lines 164, 166. Moreover, lines 164 and 166 may both be connected to a single valve through a common line connected with the outlet port 163 or 165, with check valves preventing reverse flow from the lines 128 or 133 having the higher pressure.

The hydraulic system of FIG. 2 thus includes a hydraulic motor 130 having an intake line 128 (or 134) and an output line 134 (or 128). Means 126 or 128 defines a primary path of flow to the intake line 128 or 134, respectively. Line 136 is connected with a source of hydraulic fluid defined by pump 102. Means 136, 138, and 164, or 136, 138, 142 and 146 defines an auxiliary path of flow to the intake line, which, in the illustrated embodiment, is connected with the source of hydraulic fluid defined by pump 102. Obviously, line 136 could be connected with a separate source of hydraulic fluid independent of pump 102. Flow control means 144 is responsive to the pressure in the intake line 128 and is operable to shut off communication between the intake line 128 and the auxiliary path of flow defined by lines 136, 138, 140 and 164 when the pressure in the intake line is above a predetermined minimum necessary to prevent cavitation and provide an uninterrupted stream of fluid to the motor. The flow control means 144 is operable to connect the intake line with the auxiliary path of flow when the pressure in the intake line falls below the minimum. The flow control means includes means 148 defining a valve cavity 152 having an inlet port 141 and an outlet port 163 connected in the auxiliary path of flow the outlet port 163 communicating hydraulically with the intake line 128; and the pressure responsive valve 144 controlling communication between the inlet port 141 and the outlet port 163 in response to pressure changes in the intake line 128. Valve 144 is slidable in cavity 152 such that one land 156 of the valve controls the outlet port 163, and the inlet port 141 communicates with the space between the spaced lands 154 and 156 such that the inlet port 141 communicates with the outlet port 163 when land 156 uncovers the outlet port. A port 129 connects cavity 152 with the intake line to subject land 156 to the intake pressure and spring 172 engages the other land 154 to oppose the intake pressure such that the intake pressure urges the spool valve 144 to close the outlet port 163, and spring 172 urges the spool valve to open the outlet port against the intake pressure.

While specific forms of the invention have been illustrated and described in the foregoing specification and accompanying drawings, it should be understood that the invention is not limited to the exact construction shown but various alternatives in the construction and arrangement of parts is possible without departing from the scope and spirit of the invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A hydraulic system comprising: a source of hydraulic fluid; a hydraulic motor; means defining a path of flow from said source to said motor including an intake line for said motor; means defining a path of flow from said motor including an output line for said motor; means for maintaining the pressure in the intake line at a normal value including flow control means controlling one of said paths and operable in response to pressure changes in the intake line to change the pressure in said one path until the intake pressure is returned to normal; and means providing a pilot pressure; said one path including a port in said output line controlled by said flow control means in such a manner that said port is opened and closed in accordance with the position of the flow control means relative thereto; said flow control means being urged in one direction by pressure in the intake line and in the other direction by the pilot pressure; said means providing a pilot pressure comprising a pilot chamber having restricted communication with the path of flow from said source, a pilot valve controlling communication between said pilot chamber and exhaust, said pilot valve being urged to a closed position by a force that varies in accordance with variations in the rate of flow from said source, a cavity, and wherein said flow control means comprises a spool valve received in said cavity and having a first land cooperating with one end wall of said cavity to define an inlet chamber, and a second land cooperating with a second end wall of said cavity to define said pilot chamber.

2. A hydraulic system comprising: a variable displacement pump for providing a source of hydraulic fluid; a constant displacement hydraulic motor; means defining a path of flow from said pump to said motor including an intake line for said motor; means defining a path of flow from said motor including an output line for said motor; a port in said output line; a flow control valve controlling said port and movable in one direction to open said port and decrease the resistance to flow through said output line from said motor and movable in the opposite direction to close said port and increase the resistance to fluid flow through said output line from said motor; means providing a pilot pressure independent of the pressure in said output line that increases and decreases in response to increases and decreases, respectively, in the pressure in said intake line, said flow control valve being urged in said one direction by the pressure in said intake line and in said opposite direction by said pilot pressure to thereby maintain the speed of said motor proportional to the displacement of said pump, said last named means including a pilot chamber having restricted communication with said intake line, a pilot -valve chamber, and a pilot valve controlling communication between said pilot chamber and exhaust and urged to a closed position by the pressure in said pilot valve chamber, and means connecting said pilot valve chamber with said intake line.

3. A hydraulic system as claimed in claim 1 wherein said inlet chamber communicates with the path of flow from said source and further including a passage formed in said spool valve having one end communicating with said inlet chamber and its other end in restricted communication with said pilot chamber.

4. A hydraulic system as claimed in claim 2 further including means defining a pilot valve chamber having a pressure responsive movable wall, said pilot valve being carried by said pressure responsive movable wall such that increases in the pilot chamber pressure increases the closing force on said pilot valve.

5. A hydraulic system as claimed in claim 3 further including a restricted exhaust port in said pilot valve chamber and a pilot valve chamber inlet conduit connecting said pilot valve chamber with said cavity.

6. A hydraulic system as claimed in claim 4 further including a peripheral groove in said second land communicating with said passage in the spool valve, said groove being normally in communication with said pilot valve chamber inlet conduit for conducting flow from said inlet chamber to said pilot valve chamber.

I References Cited UNITED STATES PATENTS 1,868,697 7/1932 Ellis 60-97 P 2,756,724 7/1956 Stewart et al. 9l447 3,006,323 10/1961 Tilney 9l420 3,426,648 2/ 1969 Fehlings 91---42O PAUL E. MASLOUSKY, Primary Examiner U.S. Cl. X.R. 9129, 420, 451 

